System and method for chemical potential energy production

ABSTRACT

The present invention relates to a system comprising a heat source to provide heat at the desired temperature and energy field (e.g. a solar concentrator); an electron source configured and operable to emit electrons; an electric field generator generating an electric field adapted to supply energy sufficient to dissociate gas molecules; and a reaction gas chamber configured and operable to cause interaction between the electrons with the molecules, such that the electrons dissociate the molecules to product compound and ions via dissociative electrons attachment (DEA) within the chamber.

RELATED APPLICATIONS

This application is a Continuation of PCT application Ser. No.PCT/IL2009/000743 filed on Jul. 29, 2009 which claims priority to U.S.provisional application Ser. No. 61/129,913 filed on Jul. 29, 2008 bothof which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention generally relates to a system and method for chemicalpotential energy production

REFERENCES

The following references are considered to be pertinent for the purposeof understanding the background of the present invention:

1. Rubin, R., Karni, J. and Yeheskel, J., (2004) “Chemical KineticsSimulation of High Temperature Hydrocarbons Reforming Using a SolarReactor,” J. Solar Energy Engineering 126(3), pp. 858-866.

2. Fletcher, E. A., (2001) “Solarthermal Processing: A Review,” J. SolarEnergy Engineering 123(2), pp 63-74.

3. Steinfeld, A., (2005) “Solar Thermochemical Production of Hydrogen-aReview,” Solar Energy, 78, pp. 603-615.

4. Epstein M., Olalde G, Santen S., Steinfeld A., Wieckert C, (2006)“Towards an Industrial Solar Carbothermic Production of Zinc,” 13^(th)International Symposium on Concentrating Solar Power and Chemical EnergyTechnologies, Seville, June 2006, ISBN: 84-7834-519-1, paper # FB2-S6

5. Yoneyama, H., (1997) “Photoreduction of carbon dioxide on quantizedsemiconductor nanoparticles in solution,” Catalysis Today, 39(3), pp.169-175.

6. Kaneco, S., Kurimoto H., Shimizu Y., Ohta K. and and Mizuno T.,(1999) “Photocatalytic reduction of CO₂ using TiO₂ powders insupercritical fluid CO₂,” Energy, 24(1), pp. 21-30.

7. Jun Akikusa, S.U.M.K., (2002) “Photoelectrolysis of water to hydrogenin p-SiC/Pt and p-SiC/n-TiO₂ cells,” Int. J. Hydrogen Energy 27, pp.863-870.

8. G. B. Stevens, T. Reda, B. Raguse, (2002) “Energy storage by theelectro chemical reduction of CO₂ to CO at a porous Au film”, Journal ofElectrochemical Chemistry, 526, pp. 125-133.

9. Hon Y., Ito H., Okano K., Nagasu K. and Sato S., (2003)“Silver-coated ion exchange membrane electrode applied toelectrochemical reduction of carbon dioxide”, Electrochimica Acta, 48,pp. 2651-2657.

10. Dey G R., Belapurkar A. D. and Kishore K., (2004) “Photo-catalyticreduction of carbon dioxide to methane using TiO₂ as suspension inwater”, Journal of Photochemistry and Photobiology A: chemistry, 163,pp. 503-508.

11. Eguchi K., Hochino T. and Fujihara, (1995) “Performance Analysis ofFGM-Based Direct Energy Conversion System for Space Power Applications,”Proceedings of FGM '94, edited by B. Ilschner and N. Cherradi(Polytechnic University Romandes Press, Lausanne, Switzerland, 1995),pp. 619-625.

12. Naito H., Kohsaka Y., Cooke D. and Arashi H., (1996) “Development ofa Solar Receiver for a High-Efficiency Thermionic/ThermoelectricConversion System,” Solar Energy, 58, No. 4-6, pp. 191-195.

13. Ibragimova L. B., Smekhov G D., Shatalov O. P., Eremin A. V. andShumova V. V., (2000) “Dissociation of CO₂ Molecules in a WideTemperature Range,” High Temperature, Vol. 38, No. 1, pp 33-36

BACKGROUND OF THE INVENTION

The abundant, low-cost production of potent fuels, which can be used inintrinsically clean energy processes, i.e. processes which do notproduce and emit greenhouse gases and other pollutants is a challengingtask. Steam reforming is generally used to produce hydrogen fromhydrocarbons. Steam reforming of natural gas, sometimes referred to assteam methane reforming (SMR), is the most common method of producingcommercial bulk hydrogen, as well as hydrogen, used in the industrialsynthesis of ammonia.

The steam reforming of methane and other hydrocarbons (Reaction 1below), is generally followed by a water shift reaction to convert CO toH₂. The syn-gas (i.e. synthetic mixture of hydrogen and carbon monoxide)produced in the reforming process, can also be used as enriched gasfuel, or converted to liquid fuels such as methanol. Methane CH₄ can bereformed with steam or carbon dioxide to form a mixture of carbonmonoxide and hydrogen (syn-gas) as follows:

CH₄+H₂O

CO+3H₂ ΔH=206.2 kJ/mol  (1)

CH₄+CO₂

2CO+2H₂ ΔH=247.3 kJ/mol  (2)

where ΔH is the enthalpy of the reaction. At high temperatures(700-1100° C.) and in the presence of a metal-based catalyst, steamreacts with methane to yield carbon monoxide and hydrogen.

General Description

One clean fuel production process is solar-driven methane reformingwhich has been studied extensively [1]. Reaction (2) above can bereversed to produce energy upon demand, to operate in a closed loop, andtherefore to provide a means for storage and transportation of solarenergy. Reactions with solids such as metal oxides and carbon at hightemperature [2-4] provide other solar thermo-chemical cycles for fuelproduction, without adding CO₂ to the environment.

Another example—electrolysis of water—is a simple method for clean fuel(hydrogen) production. However, it has a low attainable efficiency dueto the need of using electricity. Recently, Stoots, C. M., O'Brien, J.E., Herring, J. S., Condie,

K. G and Hartvigsen, J. J. “Idaho National Laboratory ExperimentalResearch in High Temperature Electrolysis for Hydrogen and SyngasProduction,” Proceedings of the 4th International Topical Meeting onHigh Temperature Reactor Technology HTR2008, Sep. 28-Oct. 1, 2008,Washington, DC USA have suggested performing high temperatureelectrolysis, possibly using a clean energy source, such as solarradiation. The higher temperature reduces the amount of electricityrequired for the process.

Yet another method is thermolysis—heating the substance to a temperaturewhere the free energy is equal or larger than zero and it dissociatesspontaneously [2]. Although thermolysis of water/steam or carbon dioxidedoes not require electricity, it requires very high temperatures ofabove 3000K and 2500K, respectively.

Another clean fuel production process is the photo-catalytic process,which requires neither electricity, nor high temperature. In thisprocess, a high-energy photon initiates an endothermic reaction thatproduces fuel. However, the efficiency of this method is very low (about1%) [5-7].

Multi-stage thermo-chemical processes do not require electricity andhave practical working temperatures. For example, some thermo-chemicalmethods of water decomposition can have up to 50% overallheat-to-hydrogen conversion efficiency and operate in medium-to-highworking temperatures (T<1000° C.). However, these processes are complex,and handling of rare, expensive and/or corrosive materials is required.Other multi-stage thermo-chemical processes, at higher temperature, e.g.via metal oxide reduction have also been proposed by [4], and morerecently by Diver, R. B., Siegel, N. P., Miller, J. E., Moss, T. A.,Stuecker, J. N. and James, D. L., “Development of a Cr5 SolarThermochemical Heat Engine Prototype,” Concentrated Solar Symposium,March 2008, Las Vegas, Nevada.

CO₂ electrolysis can use different metal electrodes, and liquid or solidpolymer electrolytes as shown by [8], and more recently by Stoots, C.M., O'Brien, J. E., Herring, J. S., Condie, K. G. and Hartvigsen, J. J.in the Proceedings of the 4th International Topical Meeting on HighTemperature Reactor Technology HTR2008, Sep. 28-Oct. 1, 2008,Washington, DC USA. The maximum efficiency of a non-pollutingelectrolysis system depends on the efficiency of a clean sourceelectricity system, for example, a photovoltaic-driven system. Duringelectrolysis, carbon may deposit on the electrodes, which decreasestheir efficiency, and eventually stops the process.

Stevens et. al. [8] have shown a current reduction of 40% over 100 minfor electrochemical reduction of CO₂. According to these experiments,the maximum energy storage efficiency of CO gas (as fuel) was 35%.Photo-catalytic reduction to CO in high pressure has been investigatedby Hori et al. [9] and the direct reduction of CO₂ to methane gas wasstudied by Dey et al. [10]. These processes have low rate reduction andrequire costly and/or corrosive materials.

There is a need in the art for a novel approach capable of providing anadequate solution for efficient, high rate production of clean andlow-cost synthetic fuel.

There is thus provided, according to one broad aspect of the invention,a system for producing one or more compounds with high chemicalpotential energy, the system comprising: an electron source including acathode and configured and operable to emit electrons utilizing forexample a thermionic (TI) effect; an electric field generator generatingan electric field having an energy sufficient to dissociate CO₂ and/orH₂O reactant gas molecules; and an anode spaced apart from the cathodeat a predetermined distance defining a reaction gas chamber configuredand operable to cause interaction between the electrons with CO₂ and/orH₂O gas molecules via a dissociative electrons attachment (DEA)mechanism within the chamber, such that electrons having the requiredenergy dissociate CO₂ and/or H₂O gas molecules into CO and/or H₂ and O₂.The reactant gas molecules are therefore at least one of CO₂ and H₂O andthe product compounds are O₂ and at least one of CO and H₂.

In some embodiments, the electric field generator is exposed to thermalenergy emitted from at least one of the electron source and a thermalenergy source.

In some embodiments of the invention, the system includes a thermalenergy source (heat source) configured and operable to supply thermalenergy (radiation) to the electron source thereby raising the electronsource temperature and generating thermionic (TI) electrons emissionand/or to an electric field generator (e.g. single or pluralthermoelectric devices and/or cascades, or a single or plural StirlingEngine) for generating an electric field.

Therefore, in some embodiments, the electric field generator comprisesat least one thermoelectric device and/or cascade of thermoelectricdevices and operates for utilizing temperature difference generated bythe thermal energy source. Alternatively, the electric field generatorcomprises at least one Stirling engine operating for utilizingtemperature difference generated by the thermal energy source.

In some embodiments of the invention, the system includes, instead ofjust the anode described above, an intermediate electrode adjacent to agas components separator (e.g. a membrane), both placed in between theanode and the cathode. This configuration enables (a) an additionalmeans of CO₂ or H₂O dissociation, via electrolysis, and (b) a means forseparating between the product compounds of CO and H₂ in one side, andO₂ in the other side. Therefore, the intermediate electrode isconfigured and operable to dissociate the reactant gas molecules viaelectrolysis on the surface of the separator and the gas componentsseparator is configured and operable to separate between O₂ and theother product compounds.

The inlet reactant gas is either CO₂, or H₂O or both. The CO₂, and H₂Omay be introduced into the process on the same side of the separationmembrane, or on opposite sides of it. The product compounds exiting thereaction chamber are either CO or H₂, or a mixture of both of them. Theions conducted in the membrane are either negative oxygen ions, orprotons (H⁺), or both. Oxygen molecules exit the system on the anodeside.

The present invention combines photo, thermal, electric and chemical(PTEC) processes to develop a new method, maximizing the efficiency andthe conversion rate of thermal radiation to chemical potential, in theform of CO₂ reduction to CO and O₂ and H₂O reduction to H₂ and O₂ in thesame system. The dissociation of CO₂ and H₂O may occur in the samesystem simultaneously or either one of them can be preformed alone. Theratio of CO to H₂ is controlled during the process and the mixture ofcarbon monoxide and hydrogen can be used directly as a synthesis gas(syn-gas) gaseous fuel (e.g. in power or chemical plants), or convertedto methanol or other hydrocarbons, which can be used, for example, astransportation fuels. The CO₂ and water generated during the burning ofthese fuels can be trapped, returned to the power plant and reducedagain. This method enables clean fuel production on a very large-scale,wherever thermal energy is available.

In some embodiments, the system comprises a gas components separatorconfigured and operable to separate between the oxygen ions and COand/or H₂ molecules resulting from CO₂ and/or H₂O dissociation. Theseparator may comprise a membrane configured for allowing only certaingas component such as oxygen ions (O⁻) to pass therethrough (e.g.transmitting negative oxygen ions). Such membranes may be made ofceramic material, such as for example, Yitria Stabilized Zriconia (YSZ).Its surface facing the chamber containing CO₂ has a cathode and theother surface has an anode to extract the electrons from the oxygenions, attached to a means to transfer these electrons back to thecathode.

In some embodiments of the invention, both the CO₂ and the H₂O aresupplied to the system on the cathode side of the membrane. In thiscase, the separator is used to separate O⁻ ions from the H₂ and CO; itconducts O⁻ ions from the cathode to the anode. The rates of CO₂ and H₂Odissociation are controlled by the working temperature and by the flowrate of CO₂ and H₂O entering the cathode side.

In another embodiment of the invention, the CO₂ is supplied to thesystem on the cathode side of the membrane and H₂O is supplied to thesystem on the anode side of the membrane. In this case, the separatorcan be used to separate H⁺ ions (protons) from OH⁻. The same separatorcan be used to conduct simultaneously O⁻ions from the cathode to theanode side and H⁺ ions from the anode to the cathode side. These ionconductions can be done in both directions simultaneously, or in eachdirection separately. The rates of the O⁻and H⁺ ion conductions arecontrolled by the working temperature and by the flow rate of CO₂ on thecathode side and H₂O on the anode side.

In another embodiment of the invention, the system includes a thermalenergy source (heat source) configured and operable to supply thermalenergy (e.g. concentrated sunlight radiation) to the heating elements ofat least one Stirling engine, which generates an electric field at arelatively high efficiency.

In another embodiment of the invention, the system includes a separatedmeans of generating the electric field (e.g. a separated solar electricgenerating system).

The thermal (heat) source may include a solar energy collector, whichmay for example include a set of reflectors configured to collectsunlight radiation, concentrate it and reflect it towards the electronsource.

In some embodiments of the invention, the electron source includes athermionic cathode or a photocathode. The thermionic cathode may beassociated with the electric field generator or a separate electricfield generator operable to apply an electric potential onto theelectron source, reducing the potential barrier of the cathode andenhancing the number of emitted electrons.

In some embodiments, the thermionic cathode is coated by a protectivecoating, to be protected from exposure to gaseous environment includingCO₂, CO, O⁻ and O₂. The protective coating may include an oxide metallayer, and may be configured to enable electron transmission viatunneling by reducing the work function of the cathode.

In some embodiments of the invention, the system includes a magneticfield source, operable to adjust the electron motion such that itmaximizes the probability of the electron—CO₂ dissociative attachmentreaction.

In some embodiments of the invention, the CO₂ gas is pre-heated by thegases and/or by the hot-side of the reactor walls before entering thereaction chamber. In some embodiments of the invention, the CO₂ gas isexcited by exposure to at least one of radiation electron beam (e.g.from a laser source), magnetic field, and electric field, that increasesits vibration energy as it enters the reaction chamber. This improvesthe probability of the electron—CO₂ dissociative attachment reaction.

Preferably, the system includes an electron collector configured andoperable to collect the emitted electrons, which do not combine with theCO₂ molecules.

The system of the invention is operable with high heat-to-chemicalpotential conversion efficiency, estimated to reach above 40%, and isoperable at temperatures in the range of about 600° C.-1500° C.

In some embodiments, the electron source, the electric field generator,the reaction gas chamber and the membrane are integrated in a singlemodule (e.g. cell).

According to another broad aspect of the present invention, there isalso provided a system for producing one or more compounds with highchemical potential energy. The system comprises an electron sourceincluding a cathode and configured and operable to emit electrons; anelectric field generator generating an electric field; an anode spacedapart from the cathode; an intermediate electrode and a gas componentsseparator both placed in between the anode and the cathode; theintermediate electrode being configured and operable to dissociate thereactant gas molecules via electrolysis on the surface of the separator;the reactant gas molecules being at least one of CO₂ and H₂O, theproduct compounds are O₂ and at least one of CO and H₂ respectively.

It should be noted that the system of the present invention provides oneor more product compounds having relatively high energy of formationfrom one or more chemical compounds having relatively low energy offormation. The chemical potential energy of the product compounds can betransformed to other forms of energy such as heat, work or electricityby a chemical reaction.

According to another broad aspect of the present invention, there isprovided a method for production of one or more compounds with highchemical potential energy. The method comprises supplying CO₂ (e.g. byseparating it from other combustion emission gases) and/or H₂O reactantgas molecules to a reactor including a cathode an anode and a separatorin between the anode and the cathode; applying an electric field betweenthe anode and the cathode having an energy sufficient to dissociatereactant gas molecules via a dissociative electrons attachment (DEA)mechanism and/or to reduce the reactant gas molecules by electrolysis;separating between O₂ and the other product compounds molecules; anddischarging the product compounds molecules.

The dissociation/reduction of CO₂ to CO and O₂ and of H₂O to H₂ and O₂may be carried out as follows: an electron source comprising athermionic cathode is heated by a heat source to release free electronstherefrom; electrons are emitted from the thermionic cathode using athermionic (TI) effect; an electric field is generated such as to supplyan energy field sufficient to dissociate gas molecules usingdissociative attachment effect; introducing the electrons and the gasmolecules into a reactor (e.g. reaction chamber), where the electronsdissociate gas molecules to the product compounds.

The heating of the electron source preferably includes supplying thermalenergy (e.g. solar radiation) to the electron source thereby raising theelectron source temperature and generating thermionic electrons emissionfrom the thermionic cathode. The generation of the electric field mayinclude concentration of the thermal energy and directing it onto anelectric field generator.

The thermionic (TI) effect and the electric field generation may beactivated by the same thermal energy source, e.g. a solar energyconcentrator. The latter may include collection of sunlight radiation,concentration thereof and reflection towards the electron source.

In some embodiments of the invention, gas molecules may be pre-heatedbefore their introduction into the reaction chamber. The pre-heating ofgas molecules may be performed using the same thermal energy sourceoperable to activate the thermionic (TI) effect and the electric fieldgeneration, for example using at least one heat exchanger.

The number of emitted electrons may be enhanced by applying an electricfield to the electron source.

The negative oxygen ions may be conducted through a membrane towards anelectron collector; the excess electrons released by the oxygen ions maybe combined to form O₂ molecules; and the electrons may be recycled backto the electron source. Additionally, the electrons, which did notinteract with gas molecules, may also be recycled.

In some embodiments, the electric field may be used to performelectrolysis of the gas (CO₂ and/or H₂O) on the surface of the membrane,either subsequent to or independent of the dissociative attachmentprocess. The oxygen ions are then conducted through the membrane followthe electrolysis.

The method comprises supplying the CO₂ and H₂O gas molecules on the sameside of the separator, or on opposite sides of the separator. In someembodiments, the CO₂ is introduced on the cathode side of the membrane,while the H₂O is introduced on the anode side of the membrane. In thiscase, the dissociations of CO₂ and H₂O take place on opposite sides ofthe membrane and the membrane conducts oxygen ions from the cathode tothe anode and protons (H⁺) from the anode to the cathode.

In some embodiments, the method comprises CO₂ trapping by separating CO₂from other combustion emission gases and recycling.

The method may comprise coating at least a part of the thermioniccathode to enable electron transmission via tunneling.

In other embodiments, the method comprises exposing the gas molecules toa radiation or an electrons beam, magnetic, or electric field (e.g.fluctuating field at different orientation) to increase the vibrationenergy of the gas molecules.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1A and 1B schematically illustrate a block diagram of a systemaccording to one embodiment of the present invention;

FIG. 2 schematically illustrates a flow chart of a proposed methodaccording to another embodiment of the present invention;

FIG. 3 schematically illustrates an example of configuration of a moduleof the system of the present invention;

FIG. 4 schematically illustrates one example of using the CO gas toproduce electricity via the CO₂→CO→CO₂ close cycle.

FIG. 5 shows the I-V curves in an electrolytic cell providing CO₂dissociation as a function of temperature;

FIG. 6A and 6B show thermionic electron emission in CO₂ gas at atemperature of 1150° C. (6A) and at a temperature of 1320° C. (6B); and;

FIG. 7 illustrates the concentration of CO and H₂ during tests with CO₂on the cathode side and humid air on the anode side at a celltemperature of about 650° C.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1A representing a schematic block diagram ofthe main functional elements of the system of the present invention.System 10 comprises an electron source 112 configured and operable toemit electrons, the electron source 112 is exposed to a thermal energysource 113 raising the temperature of the electron source 112; anelectric field generator 114 generating an electric field adapted tosupply an energy field sufficient to dissociate gas molecules (CO₂and/or H₂O), the electric field generator being exposed to thermalenergy flux, either from the thermal energy source 113, or from theelectron source 112, or both; a main reaction gas chamber 116 configuredand operable to cause interaction between the electrons with CO₂ and/orH₂O; such that the electrons dissociate CO₂ and/or H₂O to CO and H₂respectively and negative oxygen ions via dissociative electronsattachment (DEA) within the chamber and/or electrolysis.

The system and method of the present invention provide a low cost, highefficiency cycle that can be used on a large scale for producing fuelwithout adding CO₂ to the environment. The process involves CO₂trapping→CO₂ reduction to CO in a clean process→direct CO consumption orits conversion to other fuels (e.g. methanol) and their consumption→CO₂trapping and recycling.

The method of the invention utilizes concentrated thermal radiation(e.g. solar radiation or another heat source) for reducing CO₂ to CO andH₂O to H₂ via a dissociative electrons attachment (DEA) method andelectrolysis, using a series of coupled energy conversion steps,including (i) thermionic (TI) emission of electrons, (ii) generation ofelectric potential to supply the required energy field, using meanswhich convert thermal radiation to electrical potential, e.g.thermoelectric (TE) device, Stirling engine, etc. (iii) CO₂ and/or H₂Oreduction by the dissociative attachment of electrons,

CO₂+e⁻

CO₂ ⁻

CO+O⁻  (3a),

H₂O+e⁻

H₂O⁻

H₂+O⁻  (3b),

(iv) CO₂ and/or H₂O dissociation via electrolysis, (v) separation of COand oxygen, and (vi) de-ionization of the oxygen ions and recycling oftheir electrons. In some embodiments, at the same time, steam located onthe anode side of the membrane is dissociated and the H⁺ ions areconducted from the anode to the cathode, while O⁻ ions are conductedfrom the cathode to the anode.

Reference is made to FIG. 1B illustrating a more detailed schematicblock diagram of an example of a system, generally designated 100,configured and operable according to the invention. System 100 includesa free charged particles (electrons) source device 112 exposed to solarenergy preferably through a solar energy collector 110; an electricfield generator 114; a main reaction chamber 116; intermediate cathode117; a separator 118, and anode 120 on which the electrons are collectedand returned to the electron source 112. The system components areintegrated in a compact cell. By coupling energy conversion methods, thesystem of the present invention utilizes nearly all the input energy(minus re-radiation losses) in either its thermal, electric or chemicalconversion mechanisms.

The solar energy collector 110 may include one or more reflectors (e.g.mirrors) which collect and concentrate sunlight, and reflect it towardsthe electron source 112 to raise its temperature. As illustrated in theexample of FIG. 1B, the solar energy collector 110 may include a singlemirror shaped as a parabolic dish. It should be understood that anyother heat source that can provide the required temperature and energyflux to the electron source could be used instead of solar radiation.

The electron source may be a high efficiency electron source using thethermionic effect, operating in a CO₂/CO/O₂/O⁻ gas surrounding. Thethermionic effect refers to the thermionic emission of a flow of chargedelectrons from a heated surface, caused by thermal vibrational energy,overcoming the electrostatic forces holding electrons to the surface.The electron source may be formed by a thermionic cathode associatedwith an appropriate electric field enhancing the electron emission. Therequired temperatures may be achieved by concentrating solar radiationtowards the cathode.

The lifetime of some of these cathodes can be thousands of hours, but insome cases they should preferably be protected (e.g. by coating) fromexposure to a gaseous environment. Some of these high current cathodematerials are sensitive to carbon dioxide, carbon-monoxide, oxygen andhydrogen. The cathode may therefore be coated with a thin oxide layer ofa few nanometers to protect its surface from atoms and ions bombardment.Furthermore, the coating may enable electron transmission via tunneling,assuring that the effective work function is reduced, while protectingthe thermionic material from reaction chamber gases, namely CO₂, CO, O⁻and O₂. Several combinations of metals and their oxides may be used;such as Al/Aluminum oxide/Au or Al combinations, Ta/Tantalum oxide/Ptcombination, Titanium/Titanium-oxide, Scandium/Scandium-oxide,Zirconium/Zirconium-oxide, Tungsten/Tungsten-oxide, andHafnium/Hafnium-oxide. Other combinations like Tungsten/Scandium oxideand Lanthanum Hexaboride/oxide are also possible. Carbides, nitrides andother materials may also be used as thermionic electron sources(cathodes).

It should be noted that the number of emitted electrons is increasedwhen the electric field, e.g. between the cathode 112 and theintermediate electrode 117, is increased. The electric field reduces thepotential barrier of the cathode surface and then, more electrons canescape from the cathode surface. The applied voltage determines theelectron energy. The characteristics of the barrier, after applying theelectric field, depend on the field intensity, the distance between thecathode and the anode, the coating thickness and the dielectricconstant. The tunneling distance also depends on the electrons energy.The number of electrons passing through this oxide metal layer bytunneling depends on the oxide (insulator) layer width and on theelectric field applied on the layer. As the temperature increases, theFermi energy distribution is changed, increasing the energy of theelectrons, which increases their tunneling probability. Thecathode-anode distance should allow creation of a sufficiently highelectric field inside the insulator, but the field should not be higherthan the breakdown value. The desired electric field can be obtained byvarying the field voltage and the cathode-anode spacing.

It should also be noted that different thermionic cathode/coatingcombinations having different electron emission capability as a functionof temperature, applied voltage, electric field intensity and theadjacent medium (vacuum, inert gas, CO₂ gas) may be used. The workingtemperatures may be in the range of about 600° C.-1500° C. Each of thesethermionic cathode/coating combinations has different potential barrier,which yields different electron tunneling probability for a givenapplied electric field. For a maximum dissociative probability of theCO₂ molecule, the electron should have energy of about 4.4 eV. After therelease of the electrons, if the applied voltage is higher than needed(4.4 V), another set of decelerating electric fields can be applied toreach the needed electron energy.

Since the system operates at high temperatures (600° C.-1500° C.), theelectric field generator (e.g. a thermoelectric device or a Stirlingengine) may generate the electric field needed for the Thermo-Field(T-F) emission (i.e. the combined effect of temperature and electricfield emission). The use of a thermoelectric or a Stirling engine devicein combination with the thermionic cathode enables: (i) operation athigh temperature and (ii) use of the excess thermal energy generatedfrom the thermionic cathode to heat the thermoelectric device or aStirling engine, which generate electric potential, and then the inletgas. A Stirling engine, which has a relatively high heat-to-electricityconversion efficiency, could be used instead of the thermoelectriccascade. By properly combining the various energy conversion processes,the system efficiency is maximized.

The free electrons, energized by the electric field, enter the mainchamber, where they collide with heated reactant gas molecules (CO₂and/or H₂O). At least a portion of the thermal energy may be used toheat the reactant gas, to emit electrons from the electron source and togenerate electric potential in the electric field generator. The productcompounds leaving the system can also be used to pre-heat the incomingreactant gas. When the electrons impact the gas molecule, thedissociative attachment process described by equations (3a) and (3b)above is initiated and CO and/or H₂ are produced. The product O⁻ ionsexit chamber 116 via the separator 118, which is in the form of a gasseparation membrane (based on electric field separation). The CO and/orH₂ exit the system directly from the main chamber 116. The electronsenergy level is raised to energy cross-section required for the DEAreaction by the electrical field generator and the electron source. Theelectron energy cross section determines the probability of the processto occur. The electron energy cross sections peak at 4.4 eV, 8.8 eV and13.2 eV. Between these values, the probability drops sharply. For hightemperature (1200K), the probability of 4.4 eV and 8.8 eV are about thesame. Therefore, the preferable choice for the electron energy is thelower energy requirement (4.4 eV).

The number of electrons combining with the CO₂ and/or H₂O and creatingO⁻ ions should be maximized. Electrons, which do not combine with thegas by dissociative attachment, can react with the CO₂ and/or H₂O bymeans of electrolysis on the intermediate electrode 117. Electrons areseparated from the ions and collected on the electron collector (anode)120 and returned to the electron source 112. It is desirable that onlyelectrons from the O⁻ ions would reach that stage.

Recombination and other adverse reactions such as the followingreactions (4-6), should be avoided:

CO+O⁻

CO₂+e⁻  (4)

O⁻+2CO₂

CO₃ ⁻+CO₂  (5)

CO₃ ⁻+CO

2CO₂+e⁻  (6)

To this end, the CO must be separated from the O⁻ ions. This can be doneusing ceramics membrane materials, which allow only certain atoms (e.g.oxygen ions) to pass through them. Yitria stabilized Zriconia (YSZ) canbe used to conduct oxygen molecules for CO/⁻ separation. The oxygen ionsare then separated from the CO molecules by applying an electric field.The CO molecules continue their trajectory while the oxygen ions aredrawn through the YSZ membrane towards the anode. The drift velocity ofthe ions should allow higher diffusion rate than their production inmain chamber 116. When reaching the anode 120, the electrons aredissociated from the O⁻ ions and pass therethrough back to the electronsource 112. The oxygen is then separated from the CO. O₂ molecules areformed, once the surplus electrons are given up and exit the systemadjacent to the anode 120.

YSZ and other ceramics membranes can conduct electrons at a certaintemperature range. It should be noted that other materials, such as thecombination of Pt-YSZ, might be useful in the process together with, orinstead of YSZ. Mn/Fe-based perovskite type oxide having high oxygenpermeability at high temperature can also be used. In the DEA process,the membrane oxygen permeability rate should be higher than the O⁻ ionsproduction rate.

It should be noted that as the temperature increases, the mobility ofthe oxygen is increased. A stronger electric field also increases theions mobility and therefore the ability to separate them from the COgas. At energy levels lower than 5 eV, the probability that otherprocesses, besides (4)-(6), would occur is very low (the cross sectionfor electron attachment is too low). To have a maximum attachment crosssection, the electron energy required in the system of the presentinvention is about 5 eV.

Electrolysis can take place across the intermediate electrode 117, theseparator 118 and the anode 120 in two ways:

(a) Both CO₂ and H₂O are introduced into the main chamber 116. Then,preferably but not necessarily, following the dissociative electronsattachment (DEA), CO₂ and H₂O molecules that remain in the chamber aredissociated via electrolysis. The product O⁻ is conducted through themembrane, while the CO and H₂ exit the main chamber.(b) CO₂ is introduced to the main chamber 116, while H₂O is entered intothe chamber on the anode side of the membrane. O⁻ resulting from CO₂dissociation is then conducted through the membrane from theintermediate electrode to the anode, while H⁺ is conducted in theopposite direction, from the anode to the cathode, as illustrated inFIG. 1B. It should be understood that the conduction of O⁻ and H⁺ aretaking place in different mechanisms, which do not compete with oneanother. One the protons reach the intermediate electrode 117 they reactwith electrons and recombine to form H₂, which exits in the systemtogether with the CO.

Reference is made to FIG. 2 schematically representing an example of amethod of the invention for producing fuel, and the efficiencies of thevarious steps. In some embodiments, the method uses concentrated solarradiation for reducing CO₂ to CO via a combination of dissociativeelectrons attachment (DEA) and high temperature electrolysis. Thistechnique combines photo, thermal, electric and chemical processes tomaximize the efficiency and the conversion rate of solar radiation tochemical potential in the form of CO₂ reduction to CO and O₂. Morespecifically, an electron/electricity process occurs by photons (sunpower) interaction with a thermionic cathode, resulting in emission ofelectrons (step 1), and thermoelectric generation of an appropriateelectric field (step 2) to thereby supply the required energy field.Then, the fuel synthesis occurs, including CO₂ reduction by thedissociative attachment of electrons (eq. 3a-3b above) (step 3) andelectrolysis (step 4), followed by separation of CO and oxygen,de-ionization of the oxygen ions and recycling these electrons (step 5),resulting in the CO fuel formation. The same process can be done forwater dissociation, either separate from, or simultaneously with the CO₂dissociation process.

As indicated above, carbon monoxide and/or hydrogen can then be useddirectly as a gaseous fuel (e.g. in power or chemical plants) orconverted to methanol or hydrocarbons. The CO₂ generated during theburning of these fuels is trapped, returned to the power plant andreduced again.

Thus, according to the invention, the reduction of CO₂ to CO and O₂ isperformed via dissociative electrons attachment method (DEA) and hightemperature electrolysis, using a series of coupled energy conversionsteps.

The conversion of concentrated solar energy to free electrons withprescribed energy by combining thermionic and either severalthermoelectric stages or Stirling engine, is most efficient when theupper temperature is in the range of about 1000° C.-1400° C. Thedissociative electrons attachment (DEA) process then uses the freeelectrons, which interact with the gas molecules. Some of these stepsare actually done in mass spectrometers and similar devices that producenegative or positive ions in high vacuum. The efficiency of the processis enabled by the use of concentrated solar energy (the requiredconcentration ration would typically be larger than 2000), the use ofhigh temperature (T=1000-1400° C.), and the coupling of several energyconversion steps to minimize losses, and the use of the best fittedmaterials for the thermionic cathode and the membrane adjacent to theanode.

A similar method can be used for water splitting via reaction (3b):

Processes (3a) and (3b) are both in the gas phase and use free electronsat a specific energy range (cross section) to create a negativemolecule. The molecule is then dissociated to produce a fuel such as H₂or CO. Molecules that were not dissociated via DEA can be dissociatedvia electrolysis. Negative oxygen ions are separated from the fuel (CO &H₂), and the electrons are released and circulated back to start theprocess over.

The thermionic (TI) and electric field generations processes arearranged as a combined cycle (the region surrounded by a dash-line inFIG. 2), meaning the heat that is not utilized by TI process can be usedat a lower temperature for the electric field generator. The electronenergy and the electric potential are then used to initiate thedissociative attachment (DEA) process of pre-heated CO₂, the subsequentelectrolysis, and the separation of the O⁻ from the CO and/or H₂. Thus,the energy is converted to chemical potential in the form of fuel madeof CO and/or H₂.

The combined cycle efficiency is: η_(CC)=η_(Ti)+η_(TE)−η_(Ti)·ηTE  (⁹)

where the thermionic efficiency is marked as n, and the thermoelectricefficiency is marked as η_(TE).

The thermionic efficiency n, is defined as the electrons power, which isthe current I times the work function divided by the electron charge(i.e., expressed in Voltage units), divided by the rate of heat thatreaches the cathode:

$\begin{matrix}{\eta_{Ti} = \frac{I \cdot {\varphi/e}}{Q_{in}}} & (10)\end{matrix}$

The current is determined as I=J*A_(surf), where J is the currentdensity and is defined by the Richardson-Dushmann equation and A_(sur)is the surface area of the thermionic element. Φ is the materialeffective work function.

The thermoelectric efficiency η_(TE) is the electrical power generatedby the thermoelectric generator divided by the incoming heat rate thatreaches it from the TI cathode (e.g. via re-radiation, etc.), ordirectly from the heat source (e.g. solar radiation). The electricalwork is used to provide the electrons (from the TI cathode) with theneeded kinetic energy:

$\begin{matrix}{\eta_{TE} = {\frac{P_{({electrical\_ power})}}{Q_{TE\_ in}} = \frac{V \cdot I}{Q_{TE\_ in}}}} & (11)\end{matrix}$

The fuel production process includes the DEA, the electrolysis and theseparation processes, has an efficiency defined as:

η_(Fuel)=η_(DEA)·η_(ELEC)·η_(Sep)  (12)

where the dissociative electron attachment efficiency is marked as n DEAthe electrolysis reaction is marked as η_(ELEC), and the fuel separationefficiency is marked as η_(SEP).

The dissociative electron attachment efficiency η_(DEA) is defined asthe number of electrons participating in the DEA process (Eq. 3, above),relative to the incident electrons from the TI cathode:

$\begin{matrix}{\eta_{DEA} = \frac{N_{e\_ DEA}}{N_{TI\_ electrons}}} & (13)\end{matrix}$

The electron beam intensity along their heading direction, x, is:

N _(e) _(—) _(DEA) =N _(TI) _(—) _(electrons)·[1−exp(−N·q _(d)·x)]  (14)

where q_(d) is the DEA cross section for O⁻/CO₂ process, N is the CO₂molecules density which depends linearly on the pressure and oppositelylinearly on the temperature, and X is the electron heading direction.

The fuel separation efficiency η_(SEP) is defined as the number of COmolecules that exit the system after separation from the oxygen ionsrelative to the number of CO₂ molecules dissociated in the DEA process:

$\begin{matrix}{\eta_{SEP} = \frac{N_{CO}}{N_{{CO}\; 2{\_ dis}}}} & (15)\end{matrix}$

The total system efficiency η_(Sys) is:

η_(Sys)=η_(Heat)·η_(CC)·η_(Fuel)  (16)

where η_(Heat) is the efficiency of the heat supply to the system,η_(CC) is the combined cycle efficiency and η_(Fuel) is the fuelproduction efficiency. If the heat source is concentrated solar energy,n_(Heat)=η_(OPT), where η_(OPT) is the optical efficiency of the solarconcentrator.

The optical efficiency of the solar concentrator η_(OPT) is defined asthe power of the radiation reaching the thermionic surface, divided bythe solar radiation upon the concentrator:

$\begin{matrix}{\eta_{OPT} = \frac{Q_{in}}{Q_{Sun}}} & (17)\end{matrix}$

The maximum system efficiency can be estimated with the followingassumptions. For a solar flux of 1000 W/m², and a total current of 200 A(assuming 20 A/cm²) and assuming that ηFuel=1, η_(OPT)=0.9, thetheoretical system efficiency will be 55%. For a total current of 300 A(30 A/cm²), the maximum theoretical efficiency will increase to 84%.

It is useful to compare the efficiency of the process of the presentinvention to that of CO₂ or H₂O reduction via electrolysis only, withoutthe other components of the present invention. In the latter, themaximum efficiency depends on the efficiency of the electricityproduction. For example, if the maximum efficiency of a solar-drivenelectricity generation would reach about 40% (which is very highrelative to existing methods), the system efficiency of fuel productionvia electrolysis could then reach about 35%. Therefore, the efficiencyof the system of the present invention is considerably higher than thatof CO₂ or H₂O electrolysis. Reference is made to FIG. 3 presenting anexample of a configuration of a module of the system of the presentinvention. It should be noted that the system module 200 does notinclude a solar energy collector, although the system configurationallows its addition later. The figure illustrates the dissociation ofCO₂ and H₂O in the same system.

The system 200 includes an electron source including a thermioniccathode 202 exposed to concentrated sunlight, (an electric fieldgenerator illustrated by a thermoelectric element 214 or a cascade ofseveral such elements, or another device such as Stirling engine (e.g.free piston), an intermediate electrode 204A spaced from the cathode 202at a predetermined distance defining a main reaction chamber zone, aseparator illustrated by a YSZ membrane 206, and the anode 204B. Theanode 204B is used to separate H⁺ ions (protons) from OH⁻. The membrane206 is used to conduct O⁻ ions from the intermediate electrode 204A tothe anode 204B, and H⁺ ions from the anode 204B to the intermediateelectrode 204A. CO₂ inlet 208A and H₂O inlet 207A are provided forsupplying the CO₂ gas and the H₂O gas respectively. The inlet gases flowfirst via a first heat exchanger 210B, where they are heated by theexiting O₂, and then through a second heat exchanger 210A, where theyare heated by the existing syngas (mixture of CO and H₂). Furtherdownstream, the inlet gases enter the housing of the module, flowingadjacent to the hot internal components (e.g. 214) and are furtherheated before the CO₂ enters the main reaction zone between thethermionic cathode 202 and intermediate electrode 204A, while the H₂Oenters the reaction zone adjacent to Anode 204B. Outlet 209A exits theproduct syngas (CO and H₂) to storage via the heat exchanger 210A.Outlet 209B exits the O₂ to storage via the second heat exchanger 210B.

The distance between the intermediate electrode 204A and the cathode 202may be changed to adjust the influence of the electric field and theapplied potential. The intermediate electrode 204A enables theperformance of electrolysis and the adjustment of the electric fields inthe reaction zone (between 202 and 204A) and across the membrane(between 204A and 204B). The system 200 may be designed to operate underpressures of 0.1-10 atmospheres and at temperatures of 600° -1500° C.The anode 204 is connected to the cathode 202 to recycle the electrons.An electric field may be generated from an external power source andcontrolled independently, either in addition to or without using theelectric field generator 214. The thermoelectric (TE) or Stirlinggenerator 214 may be heated, either from an independent source, or byre-radiation from the thermionic cathode to then generate the requiredelectric potential. The cathode should have a high current density andbe able to sustain many hours of continuous work at high temperature.The current density depends on the temperature and the electric field.

Reference is made to FIG. 4 illustrating one possible way of using COgas produced in the method of the present invention. CO may be producedby using CO₂ supplied by emission of a turbine using solar energy andthe method of the present invention. After exiting the production stage,the CO is stored and used as a combustion fuel to drive the turbine andgenerate electricity. The CO₂ formed in the combustion process is thenreturned to the CO production stage, and so on.

FIGS. 5, 6 and 7 provide a sample of the data obtained from measurementswith the main components of the system of the present invention.

Reference is made to FIG. 5 illustrating laboratory measurements of I-V(Current-Voltage) curves in an electrolytic cell, configured by usingthe teachings of the present invention. The current is generated by theconduction of oxygen through the separating membrane and thereforeindicates the rate of CO₂ reduction via the electrolysis. A remarkableincrease in the reaction rate occurs as the temperature increases from640° C. (Curves D and G) to 950° C. (Curves C, E and F). A smallerincrease is seen as the temperature increases from 950° C. to 1250° C.and 1400° C. (curves B and A, respectively). The cell improvementindicated in the figure legend is primarily in the intermediateelectrode and the anode. As described above, the system of the presentinvention may combine this electrolysis cell with a thermionic cathodeand reaction chamber. Measurements with these components are presentedin FIG. 6.

Reference is now made to FIG. 6A and 6B illustrating a thermionicElectron Emission from a cathode made of CeB₆ into CO₂ gas at atemperature of 1150° C. (6A) and at a temperature of 1320° C. (6B). Thecurrent ratios refer to the electron current at a given test pressure,relative to the current at the maximum vacuum used in the measurements.The graphs show the ability of a cell based on the principles of thepresent invention to efficiently emit electrons using the thermioniceffect at a pressure of up to one atmosphere. The reduction of currentwith increasing pressure is relatively small, indicating that operationat ˜1bar is feasible.

Reference is now made to FIG. 7 illustrating the evolution of CO and H₂concentration inside an electrolytic reaction cell. The test temperaturein this case is 650° C. At the beginning of the tests, the reaction cellis evacuated and then is filled with CO₂ at about 1.1 bars. As thetemperature is increased and the electric potential is applied, the COconcentration in the cell increases over time. In this test, the cell issurrounded by air with a humidity of about 9.5gram-H₂O per 1 kg-air. Asseen in the figure, the concentration of H₂ inside the cell alsoincreases due to electrolysis of the H₂O in the surrounding air andconduction of protons through the membrane into the cell.

1. A system for gases dissociation; the system comprising: an electronsource including a cathode and configured and operable to emitelectrons; an electric field generator generating an electric fieldhaving an energy sufficient to dissociate reactant gas molecules; and ananode spaced apart from the cathode at a predetermined distance defininga reaction gas chamber configured and operable to cause interactionbetween the electrons with said reactant gas molecules via adissociative electrons attachment (DEA) mechanism within said chamber,such that electrons having the required energy dissociate said moleculesinto product compounds; said reactant gas molecules being at least oneof CO₂ and H₂O, said product compounds are O₂ and at least one of CO andH₂.
 2. The system of claim 23, comprising an intermediate electrodeadjacent to said gas components separator, both placed in between saidanode and said cathode; said intermediate electrode being configured andoperable to dissociate said reactant gas molecules via electrolysis onthe surface of said separator.
 3. The system of claim 1, wherein saidelectric field generator is exposed to thermal energy flux from at leastone of the electron source and a thermal energy source.
 4. The system ofclaim 1, comprising a thermal energy source configured and operable tosupply thermal energy to said electric field generator.
 5. The system ofclaim 4, wherein said electric field generator comprises at least onethermoelectric device and/or a cascade of thermoelectric devices, andoperates for utilizing temperature difference generated by said thermalenergy source.
 6. The system of claim 4, wherein said electric fieldgenerator comprises at least one Stirling engine operating for utilizingtemperature difference generated by said thermal energy source.
 7. Thesystem of claim 4, wherein said thermal energy source is a solar energycollector, and said thermal energy is sunlight radiation.
 8. The systemof claim 1, wherein said cathode is a thermionic cathode orphotocathode.
 9. The system of claim 8, wherein said thermionic cathodeis associated with said electric field generator or a separate electricfield generator operable to apply an electric potential between saidcathode and said anode, reducing the potential barrier of the cathodeand enhancing number of emitted electrons.
 10. The system of claim 1,wherein said system comprises a magnetic field source, operable toadjust electron motion and to maximize probability of an electron—CO₂dissociative attachment reaction.
 11. The system of claim 23, whereinthe CO₂ gas is supplied on the cathode side of the gas componentsseparator and the H₂O gas is supplied on the anode side of theseparator; said separator being configured to separate O⁻ ions from COand H⁺ ions from OH⁺ by simultaneously conducting oxygen ions from thecathode to the anode and H⁺ ions from the anode to the cathode.
 12. Thesystem of claim 23, wherein both CO₂ and H₂O gases are supplied on thecathode side of the separator; said separator being configured toseparate oxygen ions from the H₂ and CO by conducting oxygen ions fromthe cathode to the anode.
 13. A method for dissociating gas molecules;the method comprising: supplying reactant gas molecules to a reactorincluding a cathode an anode and a separator in between said anode andsaid cathode; applying an electric field between the anode and thecathode having an energy sufficient to dissociate reactant gas moleculesvia a dissociative electrons attachment (DEA) mechanism; separatingbetween O₂ and the other product compound molecules; and discharging theproduct compound molecules; said gas molecules being at least one of CO₂and H₂O, said product compound being made of O₂ and at least one of COand H₂ or a mixture of CO and H₂.
 14. The method of claim 13, comprisingdissociating gas molecules of both CO₂ and H₂O simultaneously, to CO andO₂ and to H₂ and O₂.
 15. The method of claim 13, comprising heating anelectron source comprising a thermionic cathode to release freeelectrons therefrom using a thermionic effect.
 16. The method of claim15, wherein said heating of the electron source comprises supplying athermal energy flux to said electron source thereby raising electronsource temperature and generating thermionic electrons emission fromsaid cathode.
 17. The method of claim 15, comprising supplying thermalenergy flux to an electric field generator for generating an electricfield between the anode and the cathode.
 18. The method of claim 15,comprising heating the electron source and the electric field generatorby using a same thermal energy source.
 19. The method of claim 17,comprising collecting sunlight radiation, concentrating it andreflecting it towards said electron source and/or said electric fieldgenerator.
 20. The method of claim 13, comprising applying an electricfield to the electron source to enhance number of emitted electrons. 21.The method according to claim 13, comprising supplying the CO₂ gas onthe cathode side of the separator and the H₂O gas on the anode side ofthe separator, such that the dissociations of CO₂ and H₂O take place onopposite sides of the separator; and conducting oxygen ions from thecathode to the anode and H⁺ from the anode to the cathode through saidseparator.
 22. The method according to claim 13, comprising supplyingboth CO₂ and H₂O gases on the cathode side of the separator, andseparate oxygen ions from the H₂ and CO and conducting oxygen ions fromthe cathode to the anode.
 23. The system of claim 1, comprising a gascomponents separator placed in between said anode and said cathodeconfigured and operable to separate between O₂ and the other saidproduct compounds.